Exploring the influence of B cell receptor signaling components on the development and homeostasis of B cells

Abstract

IgG1 (gamma1), known as surface marker of memory B cells, is characterized with a conserved cytoplasmic tail that is lacking in IgM (mu) and IgD (delta) chains (Geisberger et al., 2003; Neuberger et al., 1989; Reth, 1994). To investigate the physiological role of this cytoplasmic tail, a novel mouse strain (IgG1i) was established in which Cgamma1 takes place all the other CH gene segments. IgG1i mice exclusively express membrane-bound IgG1 in the B lineage. On the other hand, it is known that signal transduction through mu and delta chains relies entirely on the cytoplasmic tails of the BCR-associated Igalpha/beta heterodimer. To assess whether signal-transduction through gamma1 is also Igalpha/beta-dependent, we generated the IgalphadeltacIgG1i mice which carry the IgG1 BCR in combination with Igalpha truncation and the IgbetadeltacIgG1i mice which carry the IgG1 BCR in combination with Igbeta truncation. We found that signal of gamma1 affects peripheral B cell compartment distinctly when combining Igbeta truncation or Igalpha truncation. B cells number in IgalphadeltacIgG1i mice is over ten times as the number in Igalphadeltac mice. In contrast, IgbetadeltacIgG1i mice did not increase the number of B cells in compared with Igbetadeltac mice. Our data suggested that the increased number of peripheral B cells in the IgalphadeltacIgG1i mice compared to IgbetadeltacIgG1i mice did not come from an advantage in the BM B cell development, but rather from a peripheral influence. Due to the insufficient receptor expression, Igalpha or Igbeta mutant B cells were unable to respond to the survival signal of BAFF and exhibit down-regulated expression of basal Bcl-XL accordingly. This down-regulation of Bcl-XL is correlated with the faster faded away phosphorylation of PLCgamma2 and reduced level of IKBalpha phosphorylation. These findings suggested that the B cell survival signal through gamma1 depends on both Igalpha and Igbeta but mainly on Igbeta which is required for NF-kappaB-directed Bcl-XL expression. B cell-specific Smad7-deficient mouse Previous in vitro studies showed Smad7 is induced by the TGF-beta superfamily members and negatively modulates TGF-beta signaling. To clarify the biological role of Smad7 in Blymphocytes, we have generated a B cell-specific Smad7-deficient mouse line (CD19Cre/Smad7flox). Our data indicated that Smad7 plays an important role in TGF-beta- dependent regulation of IgA switching. Also it regulates the maintenance of B-2 B cells in the peritoneal cavity. In summary, our findings suggest that Smad7 is a crucial downstream inhibitor in TGF-beta receptor signaling in B cells.